Transmitted digital radio mondiale (DRM) signals include a succession of orthogonal frequency-division multiplexing (OFDM) symbols. Each OFDM symbol is the sum of K sine wave portions equally spaced in frequency. Each sine wave portion, called a “cell”, is transmitted with a given amplitude and phase that corresponds to a carrier position. A certain number of cells in each OFDM symbol are transmitted with a predetermined amplitude and phase and are referred to as “reference pilots.”
Reference pilots represent a certain proportion of the total number of cells. These cells are pilot cells for channel estimation and synchronization. The positions, amplitudes and phases of these cells are carefully chosen to optimize the performance, especially the initial synchronization duration and reliability. Generally, there are three types of reference cells used in DRM: frequency pilot cells, time pilot cells and gain pilot cells.
OFDM systems generally rely upon two conditions. First, the transmitted carriers and the demodulating carriers should be aligned with each other. In the second condition, the receiver performs an integrate-and-dump process over a duration equal to the reciprocal of the carrier spacing. Under these conditions, signal orthogonality holds and there is no crosstalk between carriers. If either of the two conditions does not hold, then orthogonality does not hold. Thus, some degree of crosstalk between carriers inevitably results. An error in the receiver clock frequency will cause the spacing of the demodulating carriers to differ from those transmitted. In addition, errors in the receiver clock frequency cause the duration of the receiver integrate-and-dump process to differ from the reciprocal of the transmitted carrier spacing resulting in significant crosstalk between the carriers.
The error caused by clock frequency offsets in OFDM is related to the offset and the number of sub-carriers. As the offset is increased or the number of sub-carriers is increased, the error increases. The number of sub-carriers is different for each robustness mode and frequency occupancy mode of DRM, Table 1 lists the sub-carrier number in each mode of DRM.
TABLE 1RobustnessSpectrum OccupancyMode012345A101113205229413461B 91103183207367411C———139—281D———89—179
If the stability of the receiver clock is 100 ppm, then in a worst case scenario, there should be about a 200 ppm error if it is assumed that the receiver clock stability is the same as the transmitter clock. In most cases, however, it can be assumed that there is generally better clock quality. Continuing with the example, in this condition, the worst case inter-carrier interference (dB) for each DRM mode caused by clock frequency offset between transmitter and receiver is measured and is shown in TABLE 2.
TABLE 2RobustnessSpectrum OccupancyMode012345A35.91734.86229.37328.36223.02822.039B36.89335.73230.41529.28424.09123.071C———32.937—26.506D———37.100—30.616
One example is shown in FIG. 3, an exemplary illustration 300 of a plot of the subcarrier versus the carrier 302 to inter-carrier interference (C/ICI) ratio 304. More specifically, FIG. 3 illustrates the C/ICI ratio for each of the 170 sub-carriers of robustness mode D and spectrum occupancy mode 5 for ε=1.0002 (or a 200 ppm error in sampling frequency). Accordingly, some action should be taken in the receiver part to correct such clock frequency offset in order to ensure the received program quality.
Conventional methods for clock frequency offset estimation often use pilot-assisted or time synchronization based methods. Conventional digital radio mondiale (DRM) receiver applications such as, for example, in DREAM software receivers, use pilot-assisted clock frequency offset estimation methods that uses three frequency pilots inserted into each of the orthogonal frequency-division multiplexing (OFDM) symbols. The receiver calculates the difference between two pilot frequencies and relates the difference to the desired pilot frequency. Such methods, however, exhibit a very high variance in the sample clock frequency estimation. In addition, often when DRM is used in, for example, mid-frequency/high-frequency (MF/HF) broadcasts, the received signals propagate from a poor channel condition thus the estimation result is in variably unsatisfactory.
Conventional time synchronization based clock frequency offset estimation methods typically account for the number of samples between two time indicators as shown in FIG. 1. For example, FIG. 1 illustrates system 100 where OFDM cells or symbols 102a, 102b and 102c (sometimes collectively referred to herein as OFDM symbol 102) and reference or sample points Nrecorded 104 and Nexpected 106. The clock frequency offset may be calculated according to the recorded sample number, Nrecorded 104, and expected sample number, Nexpected 106, as shown in the relationship exemplified by Equation 1 below.
                                                        T              s              ′                        -                          T              s                                            T            s                          =                                            N              expected                                      N              recorded                                -          1                                    (                  Eqn          .                                          ⁢          1                )            
In Equation 1, TS is defined as the sample interval of the transmitter (similarly, Ts′ is the sample interval of the receiver). Although FIG. 1 illustrates one possible relationship between Nrecorded 104 and Nexpected 106, it should be understood, however, that other relationships between Nrecorded 104 and Nexpected 106 may be found by, for example, using any suitable number of samples and/or symbols 102. Generally, to achieve greater accuracy, a longer observation time is required, rather than just one OFDM symbol duration (e.g., tens of symbols may be observed). However, when longer observation times are employed, the corresponding synchronization delay is greater.
There is therefore a need in the art for improved methods for sample clock frequency offset estimation in DRM.